It has been a recent tendency to remove environment polluting materials from products for protecting the global environment. In the field of optical glasses, optical glasses including components such as PbO and As2O3 which impose a heavy burden to the environment were used and produced in the past but these optical glasses have gradually been replaced by optical glasses which are free of such environment contaminating components. There are, however, still optical glasses which contain environmental contaminating components and further reduction of burden to the environment is desired for.
As SiO2—R2O—RO optical glasses having optical constants of refractive index within a range from 1.49 to 1.54 and Abbe number (ν d) within a range from 55 to 65, known in the art are optical glasses designed for sealing a stainless lens holder as disclosed in Japanese Patent Application Laid-open Publication No. Hei 3-183638, optical glasses designed for a lens fixed to a stainless holder as disclosed in Japanese Patent Application Laid-open Publication No. Hei 4-224132, optical glasses which are free of solarization as disclosed in Japanese Patent Application Laid-open Publication No. Hei 8-34633 and Pb-free crown glasses as disclosed in Japanese Patent Application Laid-open Publication No. Hei 10-167753.
Glasses containing a large amount of Al2O3 such as those disclosed in Japanese Patent Application Laid-open Publication No. Hei 3-183638 and Japanese Patent Application Laid-open Publication No. Hei 4-224132 have excellent chemical durability but poor defoaming property and require melting of raw materials at high temperature with resulting increase in the energy consumption and decrease in the productivity which is quite undesirable. Further, excessive pursuit for improving chemical durability tends to result in deterioration in processability such as polishing with resulting increase in the manufacturing cost.
The above-described Japanese Patent Application Laid-open Publication No. Hei 8-34633 discloses optical glasses which contains SnO and SnO2 as essential components for prevention of solarization. The use of SnO and SnO2, however often has the problem of affecting the melting equipment adversely because these components are alloyed with a platinum part of the melting equipment during melting of the glass with the result that leakage of glass takes place.
The Pb-free crown glasses disclosed in Japanese Patent Application Laid-open Publication No. Hei 10-167753 are free of PbO and As2O3 but contain ZnO as an essential component. The ZnO component dissolves in the form of a Zn ion into water through sludge which is produced as a result of grinding and polishing of the glass. If water containing the Zn ion is drained without being subjected to a proper treatment, it will cause water pollution and, therefore, much care must be taken in waste water treatment. From the standpoint of the environmental protection and reduction of the manufacturing cost, an optical glass which is free of an environment contaminating material to the maximum extent possible is required.
Japanese Patent Application Laid-open Publication No. Hei 9-255253 discloses Pb-free crown glasses for spectacles which are free of PbO and As2O3. These glasses, however contain a large amount of TiO2 and Nb2O5 and, for this reason, it is difficult to realize a desired ν d by these glasses.
Japanese Patent Application Laid-open Publication No. Sho 48-85613 discloses glasses for spectacles which are free of PbO and As2O3 but these glasses require chemical strengthening indispensably and contain a large amount of Al2O3. These glasses therefore have the same disadvantages as the above described glasses of Japanese Patent Application Laid-open Publication No. Hei 3-183638 and Japanese Patent Application Laid-open Publication No. Hei 4-224132.
In a hot forming such as reheat pressing, pressing of glass to a desired form at a lower temperature contributes to saving of energy and, therefore, a lower softening temperature of glass contributes to reduction of burden to the environment. Since a glass which contains a large amount of SiO2 generally has a relatively high softening temperature, it necessarily requires reheat pressing at a relatively high temperature.
It is, therefore, an object of the present invention to provide a SiO2—R2O—RO optical glass having optical constants of a refractive index (nd) within a range from 1.49 to 1.54 and Abbe number (ν d) within a range from 55 to 65 with reduced burden to the environment and at a reduced manufacturing cost.